In recent years, we have been witnessing a rapid advancement of the optical access and local area networks driven by ever growing bandwidth demand, and fundamental inability of the competing technologies, e.g. those based on twisted copper, coaxial cable or wireless transmission, to deliver. Transmission over optical fiber is emerging as a universal means for communications, from long-haul to metropolitan area to broadband access networks, resulting in an explosion of optical Internet and convergence of different media streams (e.g. data, voice, and video) into Internet Protocol data delivered in the optical domain right to the end user. This is a future proof solution to the “last mile” bottleneck, which not only dramatically increases the network capacity, but also eliminates costly transitions from optical into electrical domain (and vise versa).
Deep penetration of the optical fiber into the access networks requires an unparalleled massive deployment of the optical gear that drives the traffic to and from the Internet users. For example, optical transceivers, which receive downstream signals on one wavelength and send upstream signals on another wavelength, both wavelengths sharing the same optical fiber, have to be deployed at every optical line terminal (OLT)/network unit (ONU). Therefore, cost efficiency and volume scalability in manufacturing of such components are increasingly becoming the major issues. It is broadly accepted across the industry that the optical Internet is not going to become a commodity service, until volume manufacturing of the optical transceivers and other massively deployed optical components reaches the cost efficiency and scalability levels of consumer products.
Within a framework of the current optical component manufacturing paradigm, which is based mainly on bulk optical sub-assemblies (OSA) from off-the-shelf discrete passive and active photonic devices, the root cause of the problem is in a labor-intensive optical alignment and costly multiple packaging. Not only do these limit the cost efficiency, but also significantly restrict the manufacturer's ability of ramping volume and providing scalability in manufacturing. The solution is in reducing the optical alignment and packaging content in the OSA and, eventually, replacing the optical assemblies with photonic integrated circuit (PIC) technologies, in which all the functional elements of optical circuit are monolithically integrated onto the same substrate. Then, the active optical alignment by hand is replaced by automated passive alignment, defined by means of lithography, and multiple packaging is eliminated altogether, enabling for an automated and volume-scalable mass production of the complex optical components, based on existing planar technologies and wafer fabrication techniques.
In the context of applications, the materials of choice for monolithic PICs for use in the optical transmission systems remain indium phosphide (InP) and related III-V semiconductors, since they, uniquely, allow for active and passive devices operating in the spectral ranges of interest for optical telecommunications to be combined onto the same InP substrate. In particular, InP PICs, perhaps, are the best hope for a cost-efficient and volume-scalable solution to the most massively deployed components: optical transceivers for the access passive optical networks operating in 1.3 μm (upstream) and 1.5 μm (downstream) wavelength ranges, see for example V. Tolstikhin (“Integrated Photonics: Enabling Optical Component Technologies for Next Generation Access Networks”, Proc. Asia Optical Fiber Communication & Optoelectronic Exposition & Conference, October 2007).
In the PIC, function of every semiconductor waveguide device is pre-determined by its band structure, and, more particularly, bandgap wavelength of its guiding layer. Therefore, functionally diverse devices must be made from different, yet compatible, semiconductor materials. This is a fundamental requirement, and one that has a profound impact on the PIC design and fabrication. Integration of multiple functionalities in the PIC can be achieved in several ways varied by their design flexibility and/or fabrication complexity. Multi-guide vertical integration (MGVI) technique, in which the optical waveguides of different functionality (hence composed from different materials) are monolithically integrated one above the other in a process of epitaxial growth and coupled through evanescent fields of their optical modes, is one such technique. It is flexible, because of different optical waveguides are vertically separated and hence their guiding layers may be designed independently. Still, it is relatively easy to manufacture, since the multi-functional PIC can be fabricated by using only one epitaxial growth step and standard semiconductor fabrication processes. A combination of the design flexibility and suitability for a cost-efficient wafer fabrication makes MGVI an attractive versatile integration technique for mass production of highly functional, inexpensive optical components.
On a flip side, the design of PICs based on the MGVI platform is challenging because of a necessity for organizing of functionally different waveguide elements at different vertical levels of the MGVI structure into a common optical circuit, through a controllable transition of the optical signals between vertically stacked optical waveguides. The problem is further complicated then the PIC operates in a plurality of wavelengths, each of which is generated or processed or detected in its designated waveguide at a certain vertical level of the MGVI structure and yet all the wavelengths share the same input/output optical port. In particular, there is a need for a waveguide arrangement, hereafter referred to as a vertical wavelength (de)multiplexer (VWM), that allows for vertically combining and splitting the optical signals in the different wavelength ranges, such that, in use, signals in each particular wavelength range are transitioned from the wavelength-designated (common) input waveguide into the common (this wavelength designated) output waveguide without significantly interacting with the other wavelength-designated waveguides. Additionally, it should be compact, compliant to the PIC performance requirements and tolerant to the variations of the fabrication processes.
Early designs of what could be qualified as a VWM were based on a twin-waveguide structure, originally proposed by Suematsu et al (“Integrated Twin-Guide AlGaAs Laser with Multiheterostructure”, IEEE J. Quantum Electron., Vol. 11, pp. 457-460, 1975). This is essentially a directional coupler arrangement, in which a thin transparent layer separates two waveguides, such that, in use, optical signal of a particular wavelength and polarization is completely transferred between the two over a predetermined propagation distance, specific to the wavelength and polarization of the optical signal. Whilst very simple, this design suffers from a relatively narrow operating wavelength range and high polarization sensitivity, both related to the resonant-coupling mechanism of the transfer between the vertically stacked waveguides.
More recently, the idea of using the wavelength-selective directional coupler for a vertical wavelength splitting has received further consideration and been advanced based upon mainly resonant coupling techniques, such as resonant grating-assistant coupling (e.g. R. C. Alferness et al., “Grating-assisted InGaAsP InP vertical co-directional coupler filter”, Appl. Phys. Lett., Vol. 55, P. 2011, 1989) or resonant evanescent-field coupling. Resonant evanescent-field coupling technique itself can be sub-divided into solutions using planar waveguides (e.g. V. Magnin et al, “Design and Optimization of a 1.3/1.55-μm Wavelength Selective p-i-n Photodiode Based on Multimode Diluted Waveguide”, IEEE Photon. Technol. Lett., Vol. 17, No. 2, pp. 459-461, 2005), straight ridge waveguides (e.g. C. Wu, et al., “A Vertically Coupled InGaAsP/InP Directional Coupler Filter of Ultra-narrow Bandwidth”, IEEE Photon. Technology Lett., Vol. 3, No. 6, pp. 519-521, 1991) and tapered ridge waveguides (e.g. C.-W. Lee et al., “Asymmetric Waveguides Vertical Couplers for Polarization-Independent Coupling and Polarization-Mode Splitting”, J. Lightwave Technol., Vol. 23, No. 4, pp. 1818-1826, 2005).
Analysis of the resonant grating-assisted designs shows that these are suitable only for narrow wavelength passband applications and require that the grating is formed in the layer(s) separating the vertically integrated waveguides. This precludes the use of a one step epitaxial growth, a significant benefit of the vertical integration platform, which allows for high yield and low cost approach to manufacturing components on III-V semiconductor materials.
In the resonant evanescent-field coupling designs, the transfer between vertically integrated waveguides occurs at a pre-determined distance along the propagation axis, this position being specific to the wavelength and polarization of the optical signal. This dramatically limits a designers' freedom for designing a fully functional photonic circuit but also limits the resonant evanescent-field coupling designs only to the narrow passband applications.
Additionally, any narrow wavelength passband design requires tight fabrication tolerances, as even a minor variation of the epitaxial structure or/and layout of the device may result in a shift of centre wavelength beyond a specified passband and rendering the component useless for the intended application. This may reduce the fabrication yields and, therefore, increase the manufacturing costs of performance compliant PIG components.
Most recently, a generic approach to the VWM design suitable for applications within the MGVI platform has been proposed by V. Tolstikhin et al (“Integrated Vertical Wavelength (De)Multiplexer” U.S. patent application Ser. No. 11/882,126). Pending in this previous art, an integrated VWM operates on a principle of a lateral taper assisted adiabatic transition between a common waveguide and a plurality of wavelength-designated waveguides. All of the waveguides are vertically integrated one above the other and positioned one after the other in the order of ascending the bandgap wavelength of their guiding layers (hereafter referred to as the “bandgap wavelength”), such that the common waveguide is at the bottom of the MGVI structure and the designated waveguide corresponding to the longest bandgap wavelength is at the top of the MGVI structure. For every wavelength from the plurality of wavelengths sharing the common waveguide, wave impedance matching between this common and wavelength-designated waveguides occurs at a certain predetermined distance, such that, in use, the longer wavelengths propagate further in the common waveguide, prior to being adiabatically transferred into their designated waveguides. This is achieved through manipulating the multi-step lateral tapers, defined at each waveguide level and coherently adjusted one to the other in order to change the waveguide effective indices or, in other words, wave impedance of the waveguides, in a certain pre-determined way.
Whereas it is a generic, compact and easy to manufacture VWM design to use in the PICs based on MGVI platform, the integrated waveguide arrangement above has an inherent limitation in that the multi-step lateral tapers, a crucial element of this design needed for a controllable wave impedance change along the propagation direction, may not be necessarily compatible with the desired layout of the wavelength-designated waveguide(s) on which they are to be formed. It would be advantageous, therefore, to provide a solution that removed the constraints of this prior art by offering increased design flexibility within the MGVI platform. This would further advance it as a versatile PIC platform, based on one-step epitaxial growth and standard semiconductor fabrication processes.